Boosters help you pass levels faster.
They increase your total income for a limited amount of time.
You can apply all three types of boosters at the same time and increase the income up to x64 times!

There is the booster activation button in the bottom right corner.

If you do not have any, you can always replenish your stock in the store or use an advertisement booster.
In the bottom center there is a button using which you can increase revenue for some time by watching an ad.